jffs2: print symbolic error name instead of error code

Utilize the %pe print specifier to get the symbolic error name as a
string (i.e "-ENOMEM") in the log message instead of the error code to
increase its readablility.

This change was suggested in
https://lore.kernel.org/all/92972476-0b1f-4d0a-9951-af3fc8bc6e65@suswa.mountain/



Signed-off-by: Christian Heusel <christian@heusel.eu>
Reviewed-by: Zhihao Cheng <chengzhihao1@huawei.com>
Signed-off-by: Richard Weinberger <richard@nod.at>